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Loans (Summary Of Impaired Loans) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	$ 2,670us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment	[1]	$ 2,189us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment	[1]
Recorded Investment, With an allowance recorded	5,328us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment	[1]	5,119us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment	[1]
Recorded Investment	7,998us-gaap_ImpairedFinancingReceivableRecordedInvestment	[1]	7,308us-gaap_ImpairedFinancingReceivableRecordedInvestment	[1]
Unpaid Principal Balance, With no related allowance recorded	4,829us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance	[1]	2,661us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance	[1]
Unpaid Principal Balance, With an allowance recorded	5,328us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance	[1]	5,119us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance	[1]
Unpaid Principal Balance	10,157us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance	[1]	7,780us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance	[1]
Related Allowance	1,937us-gaap_ImpairedFinancingReceivableRelatedAllowance		2,467us-gaap_ImpairedFinancingReceivableRelatedAllowance
Average Recorded Investment, With no related allowance recorded	2,264us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment		2,445us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment
Average Recorded Investment, With an allowance recorded	5,148us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment		3,558us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment
Average Recorded Investment	7,412us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment		6,003us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Interest Income Recognized, With no related allowance recorded
Interest Income Recognized, With an allowance recorded
Interest Income Recognized
Commercial Business [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	1,461us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]	1,408us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]
Recorded Investment, With an allowance recorded	3,126us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]	2,880us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]
Unpaid Principal Balance, With no related allowance recorded	2,920us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]	1,741us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]
Unpaid Principal Balance, With an allowance recorded	3,126us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]	2,880us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	[1]
Related Allowance	1,120us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember		1,556us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember
Average Recorded Investment, With no related allowance recorded	1,384us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember		1,431us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember
Average Recorded Investment, With an allowance recorded	2,934us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember		1,998us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember
Interest Income Recognized, With no related allowance recorded
Interest Income Recognized, With an allowance recorded
Commercial Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	1,209us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]	781us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]
Recorded Investment, With an allowance recorded	2,202us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]	2,239us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]
Unpaid Principal Balance, With no related allowance recorded	1,909us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]	920us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]
Unpaid Principal Balance, With an allowance recorded	2,202us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]	2,239us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	[1]
Related Allowance	817us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember		911us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Average Recorded Investment, With no related allowance recorded	880us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember		1,014us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Average Recorded Investment, With an allowance recorded	2,214us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember		1,560us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Interest Income Recognized, With no related allowance recorded
Interest Income Recognized, With an allowance recorded
[1] Difference between recorded investment and unpaid principal balance represents partial charge-offs.

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